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Streamflow is often the only variable used to constrain hydrological models. In a previous international comparison study, eight research groups followed an identical protocol to calibrate a total of twelve hydrological models using observed streamflow of catchments within the Meuse basin. In the current study, we hypothesize that these twelve process-based models with similar streamflow performance have similar representations of internal states and fluxes. We test our hypothesis by comparing internal states and fluxes between models and we assess their plausibility using remotely-sensed products of evaporation, snow cover, soil moisture and total storage anomalies. Our results indicate that models with similar streamflow performance represent internal states and fluxes differently. Substantial dissimilarities between models are found for annual and seasonal evaporation and interception rates, the number of days per year with water stored as snow, the mean annual maximum snow storage and the size of the root-zone storage capacity. Relatively small root-zone storage capacities for several models lead to drying-out of the root-zone storage and significant reduction of evaporative fluxes each summer, which is not suggested by remotely-sensed estimates of evaporation and root-zone soil moisture. These differences in internal process representation imply that these models cannot all simultaneously be close to reality. Using remotely-sensed products, we could evaluate the plausibility of model representations only to some extent, as many of these internal variables remain unknown, highlighting the need for experimental research. We also encourage modelers to rely on multi-model and multi-parameter studies to reveal to decision-makers the uncertainties inherent to the heterogeneity of catchments and the lack of evaluation data. [ABSTRACT FROM AUTHOR]

The root zone storage capacity (Sr) of the vegetation is an important parameter for the hydrological behaviour of a catchment. Often this Sr is derived from soil and vegetation data, but a new method uses climate data to estimate Sr under the assumption that vegetation adapts its root zone capacity to overcome dry periods. This method also enables to account for the temporal variability of Sr in case of changing climate or land cover. This study applies the new method in 64 catchments in Finland to investigate the controls on Sr in boreal regions. The relations were assessed between climate derived Sr-values and detailed vegetation characteristics (leaf cover, tree length, root biomass), climate variables (precipitation-potential evaporation rate, mean annual temperature, max snow water equivalent, snow-off date) and land cover types. The results show that especially the phase difference between snow-off date and onset of potential evaporation has a large influence on the derived Sr; results even indicate that (non-)coincidence of snow melt and potential evaporation can cause a division between catchments with a high and a low Sr-value. From this study, it can be concluded that the climate derived root zone storage capacity leads to plausible results in boreal areas and that besides from climate variables, catchment vegetation characteristics can also be directly linked to the derived Sr-values. As the climate derived Sr enables incorporating climatic and vegetation conditions in a hydrological parameter, it could be beneficial to assess the effects of changing climate and environmental conditions in boreal regions. [ABSTRACT FROM AUTHOR]

International collaboration between research institutes and universities is a promising way to reach consensus on hydrological model development. Although comparative studies are very valuable for international cooperation, they do often not lead to very clear new insights regarding the relevance of the modelled processes. We hypothesise that this is partly caused by model complexity and the comparison methods used, which focus too much on a good overall performance instead of focusing on specific events. In this study, we use an approach that focuses on the evaluation of specific events and characteristics. Eight international research groups calibrated their hourly model on the Ourthe catchment in Belgium and carried out a validation in time for the Ourthe catchment and a validation in space for nested and neighbouring catchments. The same protocol was followed for each model and an ensemble of best performing parameter sets was selected. Although the models showed similar performances based on general metrics (i.e. Nash-Sutcliffe Efficiency), clear differences could be observed for specific events. The results illustrate the relevance of including a very quick flow reservoir preceding the root zone storage to model peaks during low flows and including a slow reservoir in parallel with the fast reservoir to model the recession for the Ourthe catchment. This intercomparison enhanced the understanding of the hydrological functioning of the catchment and, above all, helped to evaluate each model against a set of alternative models. [ABSTRACT FROM AUTHOR]